1. Field of the Invention
This invention relates to the carbothermic production of aluminum from a process feedstock comprising coal and clay or other oxides of aluminum associated with substantial amounts of the oxides of iron, silicon and titanium.
2. Description of the Prior Art
Many attempts have been made to produce pure aluminum by a carbothermic process to replace the universally used electrolytic process. The processes heretofore disclosed are of two general kinds. Processes of the first kind reduce substantially pure alumina, as made by the Bayer Process, with carbon to produce commercially pure aluminum. Processes of the second kind reduce clay or other mixtures comprising oxides of aluminum, iron, silicon, and titanium to make an aluminum alloy, from which commercially pure aluminum is extracted in a purification step.
Processes of the first kind have been disclosed in U.S. Pat. Nos. 2,829,961; 2,974,032; 3,723,093; 3,971,653; 4,033,757; 4,213,599; 4,216,010; 4,299,619; 4,314,846; and 4,388,107. A carbothermic process which produces commercially pure aluminum from alumina has many potential advantages over the presently used electrolytic process, including lower electrical power consumption, reduced pollution abatement costs and greater production capacity from each furnace. However such processes require pure alumina and pure carbon as feedstock and further require a process preceding carbothermic reduction to extract pure alumina from bauxite, clay or other ores containing aluminum.
Strong interest has been shown in developing processes to produce alumina from clays and other non-bauxitic sources in order to have a domestic source of raw materials in countries having insufficient bauxite reserves, such as the United States. This interest has been expressed in extensive investigations and experimentation by the U.S. Bureau of Mines and certain aluminum producers.
Processes of the second kind have been disclosed in U.S. Pat. Nos. 1,534,316; 3,257,199; and U.K. patent No. GB 2,076,022A. Processes of this kind avoid the pre-reduction steps for separation of iron, silicon and titanium compounds from the aluminum compounds in the ore, but they must provide process steps to separate aluminum from the reduction furnace alloy. These purification steps are difficult and costly and to the present time no commercial production has resulted from processes of the second kind.
One potential advantage of processes which reduce impure ore first and then extract aluminum from an alloy produced in the reduction step is that coal can be burned with oxygen to produce a large part of the overall energy to produce pure aluminum. U.K. patent No. GB 2,076,022A exemplifies this feature. However, no process has yet been disclosed which derives sufficient benefit from the ability to use coal combustion in the reduction furnace to offset the costs of extracting pure aluminum from the reduction furnace alloy.
In a report prepared for the U.S. Department of Energy, "Production of Aluminum-Silicon Alloy and Ferrosilicon and Commercial Purity Aluminum By the Direct Reduction Process", by Marshall J. Bruno, February 1983, there is described a process which burns coal and oxygen in a blast furnace to reduce clay or other oxides of aluminum, silicon, iron and titanium to an alloy comprising aluminum, iron, silicon and titanium. Commercially pure aluminum must be separated from this alloy, as for example by freeze separation of some of the silicon and further electrolytic transfer of pure aluminum from the remainder of the alloy. Separation of aluminum from such alloys by electrolytic means requires that all of the aluminum be transferred from the alloy zone to the pure metal zone.
In U.K. patent No. GB 2,076,022A there is provided a process which burns coal with oxygen to reduce mixtures of clay and alumina to produce an alloy comprising aluminum, iron, silicon and titanium. Commercially pure aluminum is said to be extracted from this alloy by selective solution in and subsequent recovery from a circulating stream of lead. Lead must be removed from the aluminum separated by this process by distillation at very low pressures and high temperatures.
Numerous other patents have issued disclosing methods to extract commercially pure aluminum from the aluminum-iron-silicon-titanium alloy produced by reduction of impure ores and impure carbon sources: for example U.S. Pat. Nos. 3,254,988; 3,257,199; 4,214,955; and 4,411,747. However, none of these has resulted in commercial operation to this date.
In U.S. Pat. No. 4,299,619 there is provided a method for production of aluminum by carbothermic reduction of alumina in a stack type reactor having a first stage in which heat is provided by combustion of carbon with oxygen and a second stage directly therebeneath which is electrically heated to complete the reaction to produce aluminum containing aluminum carbide. However, the carbon required is said to be 3.45 kg. for each kg. of aluminum produced. This carbon must be more pure than the electrode grade carbon presently used in the electrolytic process, which consumes only about 0.5 kg. of carbon per kg. of aluminum produced. This is because, in the method disclosed, all of the impurities in the carbon for heating report to the metal produced.
The extensive activities represented by the patents cited above and the work cited above to develop processes to extract alumina from non-bauxitic sources amply demonstrate the need and desirability of having a process which will achieve the potential benefits of carbothermic reduction using domestic sources of raw materials. And yet, to this date, no process has been disclosed which adequately meets this need. Processes of the first kind must employ steps to extract alumina from non-bauxitic sources which are more capital and energy intensive than the Bayer Process. Processes of the second kind must rely on alloy separation steps which are also excessively capital and energy intensive.